1. Field of the Invention
The present invention relates to communication systems. More specifically, the present invention relates to radio frequency tags.
2. Description of the Related Art
Radio frequency (RF) tags are well-known in the art. RF tags are small electronic devices that receipt and recognize radar signals of a particular type and generate and transmit a reply in response thereto. When received by the transmitting radar, the reply is decoded to ascertain the identity and location of the RF tag.
Conventional RF tags were simple repeaters designed to receive and report synthetic aperture radar (SAR) signals, shift the signal in Doppler, and transmit a reply with respect thereto using a technique known as “digital radio frequency memory” (DRFM). These devices also typically perform simple modifications of the received signal to send additional useful information.
One typical application for RF tags is to enable ground forces to communicate with airborne surveillance from standoff, i.e., non-vulnerable platforms, without being detected (low probability of detection) and homed on by their radio signals. Accordingly, the signal reply from the RF tag is typically designed to mimic reflections from surrounding terrain of radar signals from the surveillance platform.
Unfortunately, there many shortcomings associated with conventional RF tags. Firstly, the DRFM technique imposes a considerable constraint on the amount of data that can be transmitted. Secondly, there is little that can be done with a DRFM and, inasmuch as the tag is typically limited to a few bits of information, the data rates associated with the use of a DRFM are typically extremely low.
While this problem may be addressed to some extent by the use of a broadening modulation with a long pulse swept over a wide frequency band coupled with phase coding on the reply signal, this approach requires more energy for the reply signal to be decoded reliably at the aircraft. This broadening a waveform allows for the reply signal to be more easily discriminated relative to the reflections generated by the surrounding terrain and thus detected. The combination of higher energy levels and higher observeability of the reply signal renders this approach unattractive for the target application for RF tags. In addition, he higher power levels further limit battery life.
Thirdly, conventional RF tag designed is limited with respect to the types of radar signals that may be answered.
Finally, inasmuch as conventional RF tags transmit and receive data using simple analog modem techniques, these devices are susceptible to interference and dropouts.
Hence, a need remains in the art for a more robust, consistent, versatile RF tag capable of transmitting more data than conventional RF tags with a low probability of detection, while using less power.